docs: def Lockhart_Martinelli_Xtt(x, rhol, rhog, mul, mug, pow_x=0.9, pow_rho=0.5,
                              pow_mu=0.1, n=None):
      r'''Calculates the Lockhart-Martinelli Xtt two-phase flow parameter in a
      general way according to [2]_. [1]_ is said to describe this. However,
      very different definitions of this parameter have been used elsewhere.
      Accordingly, the powers of each of the terms can be set. Alternatively, if
      the parameter `n` is provided, the powers for viscosity and phase fraction
      will be calculated from it as shown below.
      
      .. math::
          X_{tt} = \left(\frac{1-x}{x}\right)^{0.9} \left(\frac{\rho_g}{\rho_l}
          \right)^{0.5}\left(\frac{\mu_l}{\mu_g}\right)^{0.1}
          
      .. math::
          X_{tt} = \left(\frac{1-x}{x}\right)^{(2-n)/2} \left(\frac{\rho_g}
          {\rho_l}\right)^{0.5}\left(\frac{\mu_l}{\mu_g}\right)^{n/2}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval [-]
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      mul : float
          Viscosity of liquid [Pa*s]
      mug : float
          Viscosity of gas [Pa*s]
      pow_x : float, optional
          Power for the phase ratio (1-x)/x, [-]
      pow_rho : float, optional
          Power for the density ratio rhog/rhol, [-]
      pow_mu : float, optional
          Power for the viscosity ratio mul/mug, [-]
      n : float, optional
          Number to be used for calculating pow_x and pow_mu if provided, [-]
  
      Returns
      -------
      Xtt : float
          Xtt Lockhart-Martinelli two-phase flow parameter [-]
  
      Notes
      -----
      Xtt is best regarded as an empirical parameter.
      If used, n is often 0.2 or 0.25.
  
      Examples
      --------
      >>> Lockhart_Martinelli_Xtt(0.4, 800, 2.5, 1E-3, 1E-5)
      0.12761659240532292
  
      References
      ----------
      .. [1] Lockhart, R. W. & Martinelli, R. C. (1949), "Proposed correlation of
         data for isothermal two-phase, two-component flow in pipes", Chemical 
         Engineering Progress 45 (1), 39-48.
      .. [2] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
         Fraction Correlations for Different Flow Patterns in Horizontal and 
         Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
         no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
      '''
      if n:
          pow_x = (2-n)/2.
          pow_mu = n/2.
      return ((1-x)/x)**pow_x * (rhog/rhol)**pow_rho * (mul/mug)**pow_mu